Although the study of mixing of granular particles in the last years has shown notable advances, it continues being moderately understood. In this work, a low shear mixing device consisting of a box with two moving walls and three static walls was used again to study granular mixing as a function of particle size. The goal is to obtain a more in depth understanding of the internal behavior of granular material in the three dimensions when particle size is changed. Experiments at different particle size distributions and wall displacements were run. Results show that faster mixing is achieved with particles of high diameter. The particle size affected the granular movement at the three directions. The phenomenon of dilation varied proportionally with the particle size. (C) 2010 Elsevier B.V. All rights reserved.